1. Field of the Invention
The present invention relates to a horizontal deflection circuit for use in a television display apparatus using a cathode ray tube (CRT).
2. Description of the Related Art
As shown in FIG. 1, a horizontal deflection circuit used in an ordinary television set is constructed such that it supplies saw tooth current to a horizontal deflection yoke while adjusting pincushion distortion by a diode modulation circuit, and a fly-back transformer which generates a high voltage to be supplied to a CRT is a load.
In general, a transistor is used as a switching element for horizontal output. Therefore, the fundamental operation of horizontal deflection in a horizontal deflection circuit using a transistor such as that shown in FIG. 1 will now be described.
In FIG. 1, when a positive horizontal driving pulse is applied to the base and a horizontal output transistor 31 is turned on, collector current flows while increasing linearly via the primary winding of a fly-back transformer 36, and positive deflection current flows through a horizontal deflection yoke 34. Next, when the horizontal output transistor 31 is turned off, the collector current becomes zero, but charging current flows from the horizontal deflection yoke 34 to resonance capacitors 33 and 53 while the combined inductance of the primary coil of the fly-back transformer 36 and the horizontal deflection yoke 34 is resonating with the resonance capacitors 33 and 53, and then discharging current for discharging it flows to the horizontal deflection yoke 34. However, since damper diodes 32 and 52 are connected to the horizontal deflection yoke 34, this resonance phenomenon stops at this stage, reverse current from the horizontal deflection yoke 34 does not flow to the resonance capacitors 33 and 53 but flows through the damper diodes 32 and 52.
The above-described operation is described below mathematically. Here, if the maximum amplitude (a peak-to-peak value, hereinafter referred to as a PP value) of horizontal deflection current I flowing through the horizontal deflection yoke 34 is denoted as Ipp, the maximum voltage of a voltage V applied across both ends of the horizontal deflection yoke 34 is denoted as Vp, the inductance of the horizontal deflection yoke 34 is denoted as L, and the horizontal retrace (hereinafter referred to as retrace) interval is denoted as Tre, then EQU V=L(dI/dt) (1)
When a retrace pulse can be approximated by a sine-wave curve, then EQU Vp=(.pi./2)LIpp/Tre (2)
Meanwhile, when the CRT to be used and the horizontal deflection yoke 34 are determined, the energy of a deflection magnetic-field required for the horizontal deflection yoke 34 to scan an electron beam is uniquely determined by the shape, high-voltage conditions, and the like of the CRT. Since the magnetic energy possessed by the current I flowing to the inductance L is (1/2)LI.sup.2, LIpp.sup.2 represents the deflection efficiency of the horizontal deflection yoke 34. If this deflection efficiency is denoted as W, EQU LIpp.sup.2 =W (3)
On the basis of equations (2) and (3), EQU IppVp=(.pi./2)W/Tre (4)
When W and Tre are constant in equation (4), the horizontal deflection current Ipp is inversely proportional to the retrace pulse voltage Vp across both ends of the horizontal deflection yoke 34.
In a horizontal deflection circuit, which has been used conventionally, such as that shown in FIG. 1, since Vp of the retrace interval is always smaller than the voltage across both ends of the switching element, Vp is limited by the switching element. Therefore, when, for example, the horizontal deflection frequency becomes twice the normal case, as in a television set free from flicker, Tre is reduced by half. Therefore, if Vp is not changed in terms of the withstand voltage performance of the switching element, Ipp becomes twice as great, thus causing a power loss in each element of the circuit to be increased. Therefore, the cost of the circuit, including each element, is inevitably increased because this power loss must be compensated for.
Also, in the conventional horizontal deflection circuit such as that shown in FIG. 1, in a case in which a high-voltage load is varied sharply, for example, in a case in which on a screen such that horizontal and vertical white lines are displayed at even intervals on a black screen called crosshatch, a screen is displayed such that the high-voltage load is very heavy while the horizontal white lines are displayed, and the high-voltage load is very light in a part where only the white vertical lines after that are displayed, since one end of the primary winding of the fly-back transformer 36 is connected to the S-shaped adjustment capacitors 55 and 35 through the horizontal deflection yoke 34, the load current variation on the primary side of the fly-back transformer 36 due to the variation of the high-voltage load appears as the variation of the current of the horizontal deflection yoke 34. This causes what is called "coging" such that the vertical lines immediately after the white horizontal lines on the screen vibrate from side to side to occur. Therefore, a damping circuit 56 for reducing the coging is connected in parallel to both ends of the S-shaped adjustment capacitor 55.